Physical properties and use data provide the basis for estimating environmental exposures to chlorpyrifos (CPY) and for assessing its risks. The vapor pressure ofCPY is low, solubility in water is <1 mg L-1, and its log Kow is 5. Chlorpyrifos has short to moderate persistence in the environment as a result of several dissipation pathways that may proceed concurrently. Primary mechanisms of dissipation include volatilization, photolysis, abiotic hydrolysis, and microbial degradation.Volatilization dominates dissipation from foliage in the initial 12 h after application,but decreases as CPY adsorbs to foliage or soil. In the days after application, CPY adsorbs more strongly to soil, and penetrates more deeply into the soil matrix,becoming less available for volatilization. After the first 12 h, other processes of degradation, such as chemical hydrolysis and catabolism by microbiota become important. The half-life of CPY in soils tested in the laboratory ranged from 2 toI ,575 d (N = 126) and is dependent on properties of the soil and rate of application.At application rates used historically for control of termites, the degradation rate is much slower than for agricultural uses. In agricultural soils under field conditions,half-lives are shorter (2 to 120 d, N=58). The mean water-soil adsorption coefficient(Koc) of CPY is 8,216 mL g-1; negligible amounts enter plants via the roots,and it is not translocated in plants. Half-lives for hydrolysis in water are inversely dependent on pH, and range from 16 to 73 d. CPY is an inhibitor of acetylcholinesterase and is potentially toxic to most animals. Differences in susceptibility result from differences in rates of adsorption,distribution, metabolism, and excretion among species. CPY is an important tool in management of a large number of pests (mainly insects and mites) and is used on a wide range of crops in the U.S. Estimates of annual use in the U.S. from 2008 to 2012 range from 3.2 to 4.1 M kg y-1, which is about 50% less than the amount used prior to 2000. Applications to corn and soybeans accounts for 46-50%of CYP's annual use in the U.S.